Don't include msg_id or reply_q in casts

On the server side, we only send replies if the request included a
_msg_id key. Also, the _reply_q key is only used when we wish to send a
reply.

So, in order to retain the exact same on-the-wire behaviour and ensure
servers aren't sending replies where none is needed, only include these
keys if we're doing a call (i.e. wait_for_reply=True).

Change-Id: Iac329493252be7d94b1ebe24f00e4d3f5c61d269
Merge "Don't include msg_id or reply_q in casts"